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A genetic study of hundreds of people within South Asia has allowed scientists to explore important transition points in the population’s history of the region, in particular pinpointing when two different groups of people mixed widely and then stopped. The study provides the genetic signature of cultural changes that occurred as the caste system was put in place in India.

Clues scattered through history
Researchers have long known that at some point in history, South Asia was a place where two separate groups of people met. The clues were to be found in the history, language, and ancient agricultural techniques and traditions, all bearing the imprint of different origins.

Sanskrit and Hindi, spoken in the north, are thought to be related to European languages, while Tamil and Telugu, spoken in the south, are unrelated to them. Agriculture in the north started earlier, some 8,000 years ago, and was distinctly related to the crops first domesticated in West Asia; farming in the south initially involved cultivation of native plants beginning around 3-5000 years ago.

The big question was when these two populations first mixed, and when did they stop?

Population interbreeding

Harvard Medical School professor of genetics David Reich specializes in analysing genetic information from modern people to understand how populations interbred in the past. This has previously revealed that present-day humans have a Neanderthal genetic material as a result of interbreeding and that the Ancestral Americans arrived in North America in successive waves.

Now, in partnership with researchers in Hyderabad, India, Reich has examined hundreds of thousands of regions in people’s genomes and found evidence that the northern and southern populations mixed around 1,900 to 4,200 years ago. That period was well after the arrival of agriculture in the region and around the same time as Indo-European languages began to be used, the researchers reported in the American Journal of Human Genetics.
“From genetic data, remarkably, you see this picture emerging of cultural change,” Reich said. The population mixture didn’t happen in pockets—it was a profound mixing that has left traces in the DNA of people in all areas of India today. But that came to an abrupt halt around 2,000 years ago, likely due to the implementation of the caste system.

Supporting evidence
Supporting evidence for the genetic interpretation comes from another source: the Rig Veda, an ancient text dating back to about 1500 BCE. Different portions of the text are thought to have been written at different times, and those parts considered the oldest contain no references to the caste system which only appear in later versions.
“The big news is that a lot of the stratification in India seems to be set down in the last few thousand years. The date estimates they give correspond to what we think is the arrival of the Indo-European languages,” said Spencer Wells, director of National Geographic’s Genographic Project, which is aimed at untangling the origins of indigenous populations. “There’s been a big debate in archaeology about how that happened.”

A new interpretation of the past

The researchers believe that instead of a completely new population invading south Asia from outside, both populations were already present in India. Thus mixing does not represent a surge of newcomers, but more likely the breakdown of some cultural or traditional barrier that had led to a natural separation between the two groups.
What most interests Reich for future research are the potential health implications of these ancient patterns of mixing. The caste system, which restricts marriage to people of different groups, gave rise to populations that were genetically isolated, and therefore may be more likely to harbour rare genetic diseases.

“That is not really well appreciated in India,” Reich said. “An important medical thing is to document this and characterise it.”